The production of synthetic or fuel gas by the gasification of various coals has become increasingly attractive due to the energy demand of the world. The gas, so produced, is withdrawn from the gasification zone and subjected to a cleaning operation to rid the gas of various contaminants which are formed or liberated from the coal during the gasification operation. These contaminants are environmental pollutants and include tars, light oils, hydrogen sulfide, ammonia, cyanide, phenols, various halogens, and particulates in the form of carbon, ash and coal as well as trace metals. The disposal and control of these contaminants present a significant drawback to the acceptance of coal gasification processes as a viable energy producer.
The gas discharged from the gasifier is normally cleansed in a scrubbing system where the gas is directly contacted with a water spray which cools the gas and condenses various condensables from the gas, such as tar, oil and organics. The water used for this scrubbing operation becomes contaminated with the tars, various organic materials and soluble gases. This waste water may be subjected to a variety of operations to clean or rid the water of various contaminants so that it may be disposed of without creating an undesirable environmental problem. Such waste water cleaning steps may include the stripping of the water to remove gases, such as hydrogen, sulfide, and ammonia. Solvent extractions may be used to remove the phenols, cyanide and inorganic anions such as halogens. However, even after such treatments the water still contains a considerable quantity of materials, especially organics which must be disposed of or somehow treated without creating an environmental pollution problem. The use of containment ponds or other disposal areas for waste water containing such potential environmental pollutants creates pitfalls due to the high chemical oxygen demand (COD) of the waste water and the possibility that these contaminated waters may enter the water table or invade surface waters.